In simple analog oscilloscopes, the onset of the sweep is provided by a trigger signal generated in reference to a particular event, which may include the transition of a predetermined threshold by the signal to be displayed. However, in digital oscilloscopes which typically quantitized the signal at discrete periodic time intervals and store signal samples for subsequent processing and display, the relationship to the trigger time and the sample interval must be consistent for the displayed image to be free of jitter or visual distortion caused by time offset of the stored signal samples. A still more difficult problem arises when the signl is continually sampled and stored before the occurrence of a trigger signal. In such cases, it is not possible to adjust the sample interval to coincide with the trigger occurrence time. Moreover, the jitter is made worse when the signal is sparsely sampled such as at a rate near the Nyquist rate.
The importance of the alignment in time of successively sampled signals is accentuated when the stored signal samples are subsequently processed according to various mathematical processes. A simple example of processes include a weighted average or a substraction between one signal and a prior signal. In such operations, for the influence of jitter in the calculated functions causes significant error in the results produced.